1. Field of the Invention
The invention relates to a corrosion-resistant clad material which has a multi-layered structure composed of aluminum alloys and is used to manufacture for example the tubes employed in various types of heat exchangers such as radiators, car heaters, intercoolers or the like.
2. Description of Prior Art
In the manufacture of such heat exchangers which must be highly corrosion-resistant, there has been used an aluminum alloy clad material comprising a core and a skin layer which is formed on at least one surface of the core by the so-called cladding process. The core is a sheet made of an aluminum alloy selected from Al-Mn alloys (i.e., aluminum-manganese alloys including "A3003" series) or Al-Mg-Si alloys (i.e., aluminum-magnesium-silicon alloys called "6000" series) which are easy to process mechanically. The skin layers serving as "sacrificially corroding layers" are made of for example an Al-Zn alloy (i.e., aluminum-zinc alloy). The core is protected from corrosion due to the sacrificial corrosion of the Al-Zn alloy forming the skin layers.
The term "sheet" in this specification is meant to include "films" which and is of a thickness less than 0.25 mm.
It is however noted that the prior art aluminum alloy clad material is disadvantageous due to its drawbacks summarized below.
In general, the constituent parts are brazed to become integral with each other to form such heat exchangers as exemplified above. Zinc atoms in the skin layer tend to diffuse into the core at high temperatures when the clad sheets used as said heat exchanger parts are brazed. With the zinc atoms which probably diffuse deep in the core, it is likely that the corrosion resistance of the core itself becomes so poor that the corrosion resistance of the heat exchanger as a whole cannot be improved to a desirable degree. Heat treatments other than the hot brazing will likewise cause such a problem.
In a case wherein a corrosive medium such as water contacts the inner surfaces of the tubes incorporated in for example a radiator, another type of clad material has been used which comprises the core of "A3003" alloy having its inner surface coated with a skin layer of another aluminum alloy "A7072". The core is easy to work, and the skin layer serves as the sacrificial corrosion layer more effectively than in the general cases as mentioned above.
However, this alternative clad material is also problematic from the following aspect.
Lighter weight and lower manufacturing cost are recently required for the tubes and other parts of the heat exchangers, and therefore efforts have been made to provide thinner parts including the tubes. A thinner core must be covered with thinner "A7072" skin layers 30 .mu.m thick or less. Such extremely thin skin layers will not produce a sufficient effect of sacrificial corrosion, thus causing the core to rapidly corrode impairing the corrosion resistance of the parts.